Conventionally, as a rubber component for a rubber composition used for manufacture of one-piece solid golf balls, solid cores of multi-layered solid golf balls (such as, two-piece solid golf balls, three-piece solid golf balls) or solid centers of thread-wound golf balls, a polybutadiene containing more than 40% of cis-1, 4 bond and having a Mooney viscosity [ML.sub.1+4 (100.degree. C.)] of less than 50, obtained by using lanthanum rare earth-based catalyst, nickel-based catalyst or cobalt-based catalyst, has been employed. This is because the polybutadiene has good workability in mixing operation using a roller, a kneader or a banbury mixer (for instance, U.S. Pat. No. 4,955,613, Japanese Kokai Publication Hei 2-80068, Japanese Kokai Publication Hei 4-109971, Japanese Kokai Publication Hei 3-151985, Japanese Kokai Publication Hei 4-73072, U.S. Pat. 5,215,308 etc.).
However, when the polybutadiene of low viscosity is used, there has been such problem that high impact resilience is difficult to obtain and consequently the golf ball with higher initial speed or higher flying distance can not be obtained.
Thus, in order to obtain a rubber composition which provides high impact resilience, it has been proposed to use a rubber having higher Mooney viscosity or higher molecular weight (for instance, Japanese Kokai Publication Sho 63-275356, Japanese Kokai Publication Sho 62-89750, Japanese Kokai Publication Hei 3-106380, Japanese Kokai Publication Hei 3-151985, etc.).
However, the use of the rubber having higher viscosity or higher molecular weight adversely gives rise to such a problem as deterioration of processability or workability. Consequently, to prevent the deterioration of processability or workability, it is also proposed to blend a rubber having lower Mooney viscosity with the rubber with higher Mooney viscosity or to blend a liquid rubber with the rubber with higher Mooney viscosity (for instance, Japanese Kokai Publication Hei 4-73072, U.S. Pat. No. 5,215,308).
These rubber compositions for golf balls are mixed by a roller, a kneader, a banbury mixer, etc. and then extruded by an extruder, followed by cutting to an appropriate size to obtain plugs for compression molding, which is used for one-piece solid golf balls, solid cores of two-piece golf balls, solid cores of three-piece golf balls, solid centers of thread-wound golf balls, etc.
For instance, in case of a rubber composition used for producing one-piece solid golf balls, it is made into cylindrical plugs having a diameter of about 32.+-.3 mm and a weight of 48.+-.2 g. In case of a rubber composition used for producing solid cores of two-piece solid golf balls, it is made into cylindrical plugs of about 29.+-.2 mm in diameter and 38.+-.2 g. In case of a rubber composition used for producing solid cores of three-piece solid golf balls or for producing solid centers of thread-wound golf balls, it is made into plugs with elliptic oval shape having about 38 mm in length, about 23 mm in width and about 21 mm in thickness.
The plugs thus obtained are usually dipped in a solution of an anti-sticking agent so that the plugs do not stick with each other and after drying, they are aged for about 8 to 48 hrs. The plugs are then charged in the respective metal mold and press-molded under heat-compression.
However, if a polybutadiene with higher Mooney viscosity is used in this process, the resulting rubber composition has poor mixing ability and poor roll workability, and provides plugs having rough surface when extruded, often showing a surface like a pinecone. When press-molded after dipping the plugs in the solution of anti-sticking agent, the anti-sticking agent enters into a gap formed before vulcanization and remains there. When the plugs are press-molded, the gap does not close and results in small creases on the surface. In the worst case, it gives cracks which cause breaking of golf balls.
Besides, in order to stabilize vulcanization, it is necessary to conduct storage the plugs after kneading and extruding and before press-molding, wherein the plugs have to be stored in a dehumidified storage chamber at a constant temperature usually for about 8 to 48 hrs, as mentioned above. However, the cis-polybutadiene tends to cause cold flow and during storage, the plugs may collapse and deform from the shape before storage, resulting in deterioration of workability when press-molding.